A hard reset is a good way to fix some software related problems. This should however be one of your last resorts, there are often less intense methods to fix many of your Android problems. Try turning your phone off for a while and then restart it, or take the battery out. Sometimes the easiest solutions are the best.

Before attempting a hard reset, ensure that you have backed up all of your data beforehand. There are a number of good apps that can do this, such as Titanium Backup, Carbon Backup and Helium.

Alternatively you can connect your LG G3 to your computer, and once the software is installed there is a one click backup button which let's you instantly back up everything, or choose the specific files you want.

To perform a hard reset:

Turn the phone off.

Press and hold the VOLUME DOWN KEY and the POWER KEY at the same time.

Release the POWER KEY only when the LG logo appears, then immediately press and hold the POWER KEY again.

RELEASE ALL KEYS when the Factory hard reset screen is shown.

Press the VOLUME DOWN key to scroll to YES, then press the POWER KEY to confirm.

Press the VOLUME DOWN key to scroll to YES, then press the POWER KEY to confirm once again.

Your device will perform a hard reset.

Or

Power off the device and wait 10 seconds
Press and hold the Volume Down button and the Power button
When the LG logo is displayed release the Power button only then immediately press and hold the Power button again.
Release both buttons when the Factory data reset screen displays.
From the Factory data reset screen, us the Power button to select Yes.